Steel, use of the steel, product made of the steel and method of producing the steel

ABSTRACT

The invention relates to a steel with a high wear resistance, high hardness and good notched bar impact strength, useful for the manufacture of products, in the use of which at least some of the features are desirable, preferably for the manufacture of tools intended to be used at temperatures up to at least 500 ° C. The steel is produced powder-metallurgically and consists in percent by weight essentially of 0.55-0.65 C, 0.7-1.5 Si, 0.1-1.0 Mn, 3.5-4.5 Cr, 1.5-2.5 Mo, 1.5-2.5 W, 1.2-1.8 V, 0-0.2 Nb, balance iron and impurities in normal amounts. After hardening and tempering the steel contains 1.5-2.5 percent by volume of MC carbides, in which M consists essentially only of vanadium, the carbides being evenly distributed in the steel matrix. The invention also relates to use of the steel, manufacture and products manufactured from the steel.

TECHNICAL FIELD

The invention relates to a steel with a high wear resistance, highhardness and good impact strength, utilizable for the manufacture ofproducts, in the application of which at least some of said features aredesirable, preferably for the manufacture of tools intended to be usedat temperatures up to at least 500° C. The invention also relates to theuse of the steel, a product made of the steel and a method of producingthe steel.

PRIOR ART

For moulding tools and machine components which are exposed to highmechanical and thermal fatigue stresses, such as moulding tools e.g. forextrusion, die-casting and for forging tools, valves and the like,hot-working steels or high-speed steels are generally used. Of thehot-working steels it is primarily steels of the type AISI H13 and ofthe high-speed steels mainly AISI M2 which are used. Both areconventional and have been known for more than 50 years. Many variationsof H13 and M2 have also been proposed and used to a certain extent, butthe classic H13 and M2 steels still predominate in their applicationareas.

BRIEF DISCLOSURE OF THE INVENTION

It is an object of the invention to provide a steel with a better wearresistance than the most common type of hot-working steels, H13. Anotherobject is that the steel shall have high hardness and toughness comparedwith dominant steel grades of the conventional type for hot-workingapplications. Yet another object is that the steel shall have high hothardness and resistance down tempering at high temperature, somethingwhich is a typical characteristic of high-speed steels, which makes thematerial suitable as hot-working steel and as a substrate for coatingusing PVD technology. An object of the invention also in this regard,however, is that the steel shall have a lower content of expensivealloying components, such as tungsten and molybdenum, than conventionalhigh-speed steels, such as high-speed steels of the M2 type. A furtherobject of the invention is that the steel shall have good workability inthe soft-annealed state of the steel and that it shall also be capableof being machined, e.g. ground, in the hardened state.

These and other objects can be achieved therein that the steel isproduced powder-metallurgically, that it has a chemical composition asstated in appending claim 1 and that it contains 1.5-2.5 percent byvolume of MC carbides, in which M consists essentially only of vanadium,said carbides being evenly distributed in the matrix of the steel.

The powder-metallurgical production of the steel can be carried out byapplying known technology to produce steel, preferably by using theso-called ASP® process. This comprises the production of a steel meltwith the chemical composition intended for the steel. Powder is producedfrom the melt in a known manner by gas-atomisation of a stream of moltenmetal, i.e. by deintegrating it into small drops by means of jets ofinert gas, which are directed at the stream of molten metal, which dropsare rapidly cooled so that they solidify to form powder particles duringfree fall through the inert gas. Following screening, the powder isinserted into capsules, which are cold-compacted and then exposed to hotisostatic compaction, so-called HIP-ing, at high temperature and highpressure to full density. HIP-ing is typically carried out at anisostatic pressure of 900-1100 bar and a temperature of 1000-1180° C.,preferably 1140-1160° C.

With reference to the contents of the various alloying components in thesteel, the following applies.

Vanadium shall be present in a content of at least 1.2% and max. 1.8% inorder together with carbon to form 1.5-2.5 percent by volume of MCcarbides in the steel. The powder-metallurgical production processcreates the conditions for these carbides to acquire the form of smallinclusions of essentially equal size with a typically round or roundedshape and even distribution in the matrix. The maximum size of the MCcarbides, reckoned in the longest length of the inclusions, is 2.0 μm.More precisely, at least 90% of the total carbide volume consists of MCcarbides with a maximum size of 1.5 μm, and more precisely thesecarbides have a size which is greater than 0.5 but less than 1.5 μm. TheMC carbides can also contain a small quantity of niobium. Preferably,however, the steel is not deliberately alloyed with niobium, in whichcase the niobium carbide element in the MC carbides can be disregarded.As well as carbon, a small quantity of nitrogen can also combine withvanadium to form the hard inclusions, which are here designated MCcarbides. However, the nitrogen content in the steel is so small thatthe nitrogen component in the inclusions does not prompt the designationvanadium carbonitrides, but can be disregarded. The content of vanadiumamounts preferably to 1.3-1.7%. The nominal vanadium content in thesteel is 1.5%.

Carbon shall be present in the steel in a sufficient quantity to combineon the one hand with vanadium to form MC carbides in the above quantity,and on the other hand to be present dissolved in the matrix of the steelin a content of 0.4-0.5%. The total content of carbon in the steel shalltherefore amount to 0.55-0.65%, preferably to 0.57-0.63%. The nominalcarbon content is 0.60%.

Silicon shall be present in the steel in a minimum content of 0.7%,preferably at least 0.85%, to contribute to the hot hardness of thesteel and its resistance to tempering during use. However, the contentof silicon must not exceed 1.5%, preferably max. 1.2%.

Manganese is not a critical element in the steel according to theinvention but is present in a quantity of between 0.2% and 1.0%,preferably in a content of between 0.2% and 0.5%.

After hardening and tempering, the steel according to the invention doesnot contain any notable content of chromium carbide, e.g. M₇C₃- orM₂₃C₆-carbides, which normally occur in hot-working steels. The steelaccording to the invention may therefore contain a max. of 5% chromium,preferably a max. of 4.5% chromium. However, chromium is in itself adesirable element in the steel and shall be present in a minimum contentof 3.5%, preferably at least 3.7%, in order to contribute to thehardenability of the steel and together with molybdenum, tungsten andcarbon to give the martensitic matrix of the steel in the hardened statethe character of a high-speed steel, i.e. a good combination of hardnessand toughness. The nominal chromium content is 4.0%.

Molybdenum and tungsten shall both be present in the steel, preferablyin roughly equal amounts in order together with carbon and chromium togive the matrix of the steel its features just stated. Tungsten andmolybdenum also contribute to counteract decarburization when they arecorrectly balanced relative to one another. Molybdenum and tungstenshall therefore each be present in a content of at least 1.5% and max.2.5%, preferably in a content of between 1.7 and 2.3%. The nominalcontent is 2.0% for both molybdenum and tungsten.

Nitrogen is not added deliberately to the steel but can occur in acontent of from 100 to 500 ppm.

Oxygen is an unavoidable impurity in the steel but can be toleratedowing to the powder-metallurgical production process of the steel inamounts up to 200 ppm.

Other impurities, such as sulphur and phosphorus, can occur and betolerated in amounts, which are normal for hot-working steels andhigh-speed steels. This also applies to impurities in the form ofmetals, such as tin, copper and lead, which are not dissolved in theaustenite in the austenitic state of the steel, and which areprecipitated following solidification, as the austenite grains areformed at high temperature, said impurities being distributed over alarge surface, as the austenite grain size is small, wherebyconcentrations of these impurities are countered, which renders theimpurities harmless. However, the steel according to the inventiontypically does not contain impurity metals of the type tin, copper andlead in amounts of more than 0.10, 0.60 and 0.005% respectively and intotal not more than a max. of 0.8% of said or other undesirable impuritymetals.

The products for which the steel is intended to be used can be worked tonear final shape, which can be carried out in a conventional manner, bymeans of cutting machining, e.g. milling, drilling, turning, grindingetc. or by means of spark machining in the soft-annealed state of thesteel. In its soft-annealed state, the steel has a hardness of 230 HBmax. (Brinell hardness), which can be obtained by soft-annealing of thesteel at 850-900° C. and then cooling to room temperature, with at leastthe cooling from the soft-annealing temperature down to 725° C., andpreferably down to at least 700° C., being carried out as slow,controlled cooling at a cooling rate of 5-20° C./h, preferably at acooling rate of approx. 10° C./h. Cooling to room temperature from atleast 700° C. or a lower temperature can take place by means of freecooling in air.

After hardening and tempering, the steel according to the invention hasa hardness of 50-59 HRC (Rockwell hardness) and an impact strengthcorresponding to an absorbed impact energy of 150-300 Joule in an impacttest using an un-notched test specimen with the dimensions 7×10×55 mm,and a structure of tempered martensite containing said MC carbidesevenly distributed in the martensite, obtainable through hardening ofthe product from an austenitization temperature of between 950 and 1160°C., cooling to room temperature and tempering at 540-580° C. Dependingon what the object produced from the steel is to be used for, i.e. theapplication range of the steel, an optimal hardness is selected in thehardness range 50-59 HRC. For hot-working applications, e.g. forhot-working rolls, forging tools and dies and other parts for theextrusion of aluminium, the optimum hardness range is between 52 and 58HRC, taking the desired good impact strength into consideration. Ahardness in said range can also be optimal for machine componentsintended to work at room temperature or at a temperature up to 500° C.,although hardnesses down to 50 HRC can also be acceptable for this typeof products. The steel according to the invention can however also beused for cold-working tools and wear parts, in which case an optimalhardness can be 56-59 HRC, possibly at the expense of a certainreduction in impact strength at hardnesses up to 59 HRC. The desiredhardness in said ranges is achieved by the choice of austenitizationtemperature in the range 950-1160° C. according to the principle “thehigher the austenitization temperature, the greater the hardness”, andvice-versa.

Further features and aspects of the invention are evident from theclaims and from the following description of experiments carried out.

BRIEF DESCRIPTION OF DRAWINGS

In the following description of experiments carried out, reference willbe made to the accompanying drawings, of which

FIG. 1 shows in the form of a diagram the impact strength versus thehardness at room temperature for a number of steels investigated,

FIG. 2 is a diagram showing the wear resistance in relation to thehardness of a steel according to the invention and of a couple ofreference materials, and

FIGS. 3-4 show in the form of a diagram the resistance to tempering at550 and 600° C. respectively for the steel alloys G and H13.

DESCRIPTION OF EXPERIMENTS CARRIED OUT

The chemical composition in percent by weight of the steel alloysinvestigated and the content of MC carbides in percent by volume of thematerials produced powder-metallurgically are shown in Table 1.

TABLE 1 Chemical composition in percent by weight of steel alloysinvestigated and content of MC carbides in pereent by volume MC O Ncarbides Alloy C Si Mn Cr Mo W V Nb ppm ppm % by vol. A 0.61 0.46 0.353.99 1.99 1.99 1.72 0.00 n.a. 290 1.8 B 0.67 0.48 0.36 4.01 2.00 2.012.06 0.01 n.a. 280 2.3 C 0.72 0.48 0.36 3.99 2.00 1.99 2.04 0.00 92 2902.5 D 0.75 0.48 0.34 3.98 2.00 2.00 2.05 0.00 89 300 2.8 E 0.48 0.490.33 4.00 1.98 1.98 1.04 0.00 70 260 0.7 F 0.55 0.49 0.32 4.00 2.00 2.071.08 0.51 67 230 1.7 G 0.60 1.00 0.32 4.02 1.99 2.06 1.51 0.01 62 3502.3 H13 0.60 0.47 0.32 3.99 3.03 3.03 1.05 0.01 n.a. n.a. — A1S1/M2*0.85 0.30 0.30 4.00 5.00 6.00 2.00 — — — — n.a. not analysed *nominalcomposition

Steel alloys A-G were produced powder-metallurgically according to theASP (ASEA-STORA-Powder) process in the following way. Approx. 300 kg ofpowder was produced from each of the alloys by nitrogen gas atomisationof a steel melt. Approx. 175 kg of the powder was enclosed in a sealedmanner in a sheet metal capsule, diameter 200 mm, length 1 m, bywelding. The capsule was placed in a hot isostatic press, HIP, withargon gas as the pressing medium, and exposed to a high pressure andhigh temperature, 1000 bar and 1150° C. respectively, for approx. 1 h.Following consolidation of the powder, so-called HIP-ing, to form acompletely dense steel body without any porosity, the capsule and itscontents were allowed to cool slowly, 10° C./h from approx. 900 toapprox. 700° C. (soft-annealing) in order to be able to be worked bysawing. The chemical composition of the steel was analysed both fromsamples from the melt and from material sawn from the capsule (Table 1).In the next stage, all capsules were forged down to a diameter of 100 mmand further by forging and rolling in several steps to a final dimensionof 9×12 mm.

The steel alloy H13 was conventionally produced hot-working steel of themodified AISI H13 type, while the last.steel in the table was aconventional high-speed steel of the type AISI M2.

A number of test specimens of the dimensions 7×10×55 mm were producedfrom steel alloys A-G. The test specimens were hardened by heating atsix different temperatures, namely between 950° C. and 1180° C., throughheating at said temperatures, cooling to room temperature and tempering3×1 h at 560° C. The hardness and impact strength of un-notched testspecimens were then measured at room temperature. The results are shownin Table 2 and 3 and in the diagram in FIG. 1.

TABLE 2 Hardness (HRC) after hardening from 950 to 1180° C. andtempering 3 × 1 h at 560° C. for alloys A-G Hardening temperature A B CD E F G  950 50.7 51 51.6 51.8 51.1 50.7 51 1000 52.3 52.6 53.2 53.252.4 52.9 52.3 1050 54.7 54.6 55 55.5 54.6 55.2 54 1100 56.5 56.7 5757.4 57.3 58.6 56.5 1150 59 59 59.6 59.5 58.6 60.8 58.3 1180 59.4 60 6161.1 59.8 61.5 59.5

TABLE 3 Impact strength (Joule) after hardening from 950 to 1180° C. andtempering 3 × 1 h at 560° C. for alloys A-G Hardening temperature A B CD E F G  950 285 282 274 266 289 275 260 1000 285 294 278 248 294 269272 1050 290 294 294 284 294 289 283 1100 287 283 264 251 294 278 2851150 167 179 164 156  92 142 258 1180 169  91  95  93  38  30 204

Table 2 and 3 and FIG. 1 show that a good impact strength was achievedfor steel alloy G in a wide hardness range and in particular in thehardness range which is particularly interesting, in particular forhot-working applications and to a certain extent also for cold-workingtools and for wear parts, namely the hardness ranges 52-58 HRC and 56-59HRC, respectively. It is true that steel alloy F had an even bettercombination of hardness and impact strength in a wide hardness range,but this steel on the other hand contains only 1.7 percent by volume ofMC carbides, which is too little to give the desired wear resistance.

Hardness and impact strength were also measured for the same steelalloys after hardening from three different temperatures between 1000and 1100° C. and tempering 3×1 h at 540° C. The results of thesesupplementary measurements are found in Table 4 and 5 and confirm thetendencies from the heat treatment, which included tempering at asomewhat higher temperature.

TABLE 4 Hardness (HRC) after hardening from 1000 to 1100° C. andtempering 3 × 1 h at 540° C. for alloys A-G Hardening temperature A B CD E F G 1000 52.9 53.9 53.6 54.1 53 53.5 53 1050 55.1 55.3 55.9 56.4 5556.4 55.4 1100 57.9 57.9 58.5 59.1 58.1 59.8 57.7

TABLE 5 Impact strength (Joule) after hardening from 1000 to 1100° C.and tempering 3 × 1 h at 540° C. for alloys A-G Hardening temperature AB C D E F G 1000 289 294 287 281 294 287 263 1050 291 284 280 273 288289 264 1100 291 269 249 253 294 258 287

The wear resistance was measured for the reference materials H13 andAISI M2 and were compared with the wear resistance of the steelaccording to the invention, steel alloy G, which was hardened from atemperature of 1150° C. and which after tempering 3×1 h at 560° C.acquired a hardness of 58 HRC. The wear resistance measurements wereperformed in a pin-on-disc test with dry SiO₂ paper type 00, with asliding rate of 0.3 m/s, load 9 N, sample dimension 3×5×30 mm. As isclear from the diagram in FIG. 2, the material according to theinvention, alloy G, had a considerably better wear resistance than theknown hot-working steel H13. The highest wear resistance was noted forAISI M2, but the difference compared with alloy G is remarkably small inview of the considerably higher content of qualified alloying elementsin the high-speed steel AISI M2.

The resistance to tempering was also studied, i.e. the dependence of thehardness on temperature and time, for alloys G and H13. The tests werecarried out at 550 and 600° C. for 1-100 h. The results are shown in thediagrams in FIGS. 3 and 4, which show that the hardness for alloy Gdeclines more slowly than for alloy H13 with time.

In optical microscope examinations of alloy G no carbides other than MCcarbides could be noted and no MC carbide larger than 2.0 μm. Of thecarbides, which could be observed in the optical microscope examination,at least 90 percent by volume were judged to be of a size greater than0.5 but less than 1.5 μm.

What is claimed is:
 1. Steel with a high wear resistance, high hardnessand good impact strength, useful for manufacturing products, wherein thesteel is produced powder-metallurgically, that it essentially consistsin percent by weight of 0.55-0.65 C  0.7-1.5 Si  0.1-1.0 Mn  3.5-4.5 Cr 1.5-2.5 Mo  1.5-2.5 W  1.2-1.8 V   0-0.2 Nb

balance iron and impurities in normal amounts, and that the steel afterhardening and tempering contains 1.5-2.5 percent by volume of MCcarbides, in which M consists essentially only of vanadium, saidcarbides being evenly distributed in the steel matrix.
 2. Steelaccording to claim 1, wherein said MC carbides have an essentially roundor rounded shape with a maximum extension of 2.0 μm.
 3. Steel accordingto claim 2, wherein at least 90 percent by volume of said MC carbideshave a size greater than 0.5 μm but less than 1.5 μm.
 4. Steel accordingto claim 1, with a chemical composition wherein the steel, in percent byweight, contains 0.57-0.63 C.
 5. Steel according to claim 4, wherein itcontains not more than impurity level content of niobium.
 6. Use of asteel which is produced powder-metallurgically and which essentiallyconsists in percent by weight of 0.55-0.65 C  0.7-1.5 Si  0.1-1.0 Mn 3.5-4.5 Cr  1.5-2.5 Mo  1.5-2.5 W  1.2-1.8 V   0-0.2 Nb

balance iron and impurities in normal amounts, and which after hardeningand tempering contains 1.5-2.5 percent by volume MC carbides, in which Mconsists essentially only of vanadium, said carbides being evenlydistributed in the steel matrix, the use comprising the step of:manufacturing the type of products which include tools and machinecomponents and which are intended to be used at temperatures up to 500°C.
 7. Use according to claim 6 for manufacturing said products, whichafter working in the soft-annealed state of the steel to at least nearfinal shape and hardening have a hardness of 50-59 HRC (Rockwell Chardness) and an impact strength corresponding to an absorbed impactenergy of 150-300 Joule in impact testing using an un-notched testspecimen with the dimensions 7×10×55 mm and a structure of temperedmartensite containing said MC carbides evenly distributed in themartensite, obtainable by hardening of the product from austenitizationtemperatures between 950 and 1160° C., cooling to room temperature andtempering at 540-580° C.
 8. Use according to claim 6, for manufacturingsaid products of said steel by working in the soft-annealed state of thesteel, in which the steel has a hardness of max. 230 HB (Brinellhardness), which condition is obtainable by soft-annealing of the steelat 850-900° C. and then cooling to room temperature, wherein at leastthe cooling from the soft-annealing temperature down to 725° C. iscarried out as slow, controlled cooling at a cooling rate of 5-20° C./h.9. Use according to claim 6, said MC carbides having a maximum extensionof max. 2.0 μm, at least 90 percent by volume of the MC carbides havinga size which is greater than 0.5 μm but less than 1.5 μm.
 10. Method ofproducing a steel with a high wear resistance, high hardness and goodimpact strength, useful for manufacturing products, in the applicationof which at least some of said features are desirable, preferably formanufacturing tools intended to be used at temperatures up to at least500° C., wherein a steel melt is produced, which essentially consists inpercent by weight of 0.55-0.65 C  0.7-1.5 Si  0.1-1.0 Mn  3.5-4.5 Cr 1.5-2.5 Mo  1.5-2.5 W  1.2-1.8 V   0-0.2 Nb

balance iron and impurities in normal amounts, comprising the steps of:disintegrating the steel melt into small drops by gas atomization;cooling the drops to form powder particles; enclosing gas-tightly thepowder particles in a sheet metal capsule; and consolidating the powderparticles to form a completely dense steel body by means of hotisostatic pressing.
 11. Method of manufacturing a product of a steelproduced according to claim 10, further comprising the steps of:hot-working by forging and/or hot rolling the hot-isostatically pressedbody; soft-annealing the steel at 850-900° C.; cooling the steel to roomtemperature by controlled cooling to a hardness of 230 HB max. (Brinellhardness); working the steel in its soft-annealed state to at least nearfinal shape; hardening the steel from a temperature between 950 and1160° C.; cooling the steel to room temperature; and tempering the steelat 540-580° C., due to which heat treatment the steel is caused tocontain 1.5-2.5 percent by volume of MC carbides, in which M consistsessentially only of vanadium, said carbides being evenly distributed inthe steel matrix.
 12. Product, wherein it is manufactured according toclaim
 11. 13. Steel according to claim 1, with a chemical compositionwherein the steel, in percent by weight, contains 0.85-1.2 Si.
 14. Steelaccording to claim 1, with a chemical composition wherein the steel, inpercent by weight, contains 0.2-0.5 Mn.
 15. Steel according to claim 1,with a chemical composition wherein the steel, in percent by weight,contains 3.7-4.3 Cr.
 16. Steel according to claim 1, with a chemicalcomposition wherein the steel, in percent by weight, contains 1.7-2.3Mo.
 17. Steel according to claim 1, with a chemical composition whereinthe steel, in percent by weight, contains 1.7-2.3 W.
 18. Steel accordingto claim 1, with a chemical composition wherein the steel, in percent byweight, contains 1.3-1.7 V.
 19. Steel according to claim 1, wherein saidMC carbides have an essentially round or rounded shape with a maximumextension of 1.5 μm.
 20. Use according to claim 6, said MC carbideshaving a maximum extension of max. 1.5 μm, at least 90 percent by volumeof the MC carbides having a size which is greater than 0.5 μm but lessthan 1.5 μm.